In recent years a new forging process known as warm forging has begun to evolve. It holds the promise of applying the well known advantages of conventional forging to applications for which conventional forging is considered too costly; specifically it appears highly likely that warm forging can be economically used to manufacture parts which are currently produced by casting and pressing techniques, both of which have been considered to be substantially less costly than conventional forging for many end applications.
It will be understood that the terms "hot forging", "warm forging" and "cold forging" are used in the shaping industry today. In the following description and claims, no reference will be made to "cold forging" which is essentially forging at room temperature, and the term "hot forging" will be used synonymously with "conventional forging".
Hot or conventional forging as currently practiced in the steel industry may be broadly described as the shaping of steel work pieces which have been heated, prior to shaping, to a temperature above the transformation temperature and which are then shaped, or formed, while at a temperature above the transformation temperature. Typical conventional forging temperatures of work pieces are in the range of 2200.degree. F. to 2350.degree. F., though in the majority of applications temperatures near the lower end of the range are employed.
Warm forging, in contrast, as currently practiced is carried out at temperatures below the transformation temperature. A typical warm forging temperature range as currently employed in the relatively small number of applications in which the technique is used may be on the order of from about 1400.degree. F.-1800.degree. F.
Warm forging has a number of advantages over conventional forging.
In warm forging up to about 98% of the metal work piece which is placed in the dies preparatory to forming is recovered in the forged part. As a consequence, far less post-forging operations, such as costly machining operations, must be performed on the work piece to bring it to its final desired size. This process of forging close to final required dimensions has been described as "precision forging" or "near net shape forging" or simply "near net" forging. This high utilization factor of warm forging is in contrast to conventional forging in which only about 70% of the material comprising the original work piece becomes the final product. As a consequence the approximately 30% of unutilized material in the work piece becomes "flash" which must either be disposed of or, more usually, is recycled by being sold back to steel makers at a small fraction, which may be on the order of about 10%, of its original cost. Hence warm forging results in a significant work piece material savings in both cost of work piece material required and subsequent scrap handling.
The fact that the resulting forging is closer to final size than the same work piece produced by conventional forging reduces the ultimate cost by eliminating all, or nearly all, of the post-forging special machining costs which are required in conventional forging to bring the work piece into specification.
Further, since the work piece is only heated to about 1500.degree. F., as contrasted to the 2200.degree.-2350.degree. F. temperature used in conventional forging, a large energy saving results, not only on the basis of the final poundage of the forged product, but also on the basis of heat consumption of flashing.
Further, with respect to heat considerations, the problem of over heating of the work piece, or of the dies when a work piece becomes stuck therein, is substantially or entirely eliminated.
In addition, better surface quality is achieved by warm forging in many cases. In particular, warm forging results in less surface scale on the work piece. This decrease in surface scale results in better die life, cleaning forged work pieces, and less post forging clean up of the forged work piece.
The foregoing advantages do however put new demands on the forming equipment, and most particularly the forming dies or other components which come into direct contact with the work piece or are directly impacted thereby.
One of the more critical requirements is the dramatic increase in pressure, or probably more accurately, force applied to the die or other forming surfaces in warm forging as contrasted to conventional forging. It is for example, easier to shape a work piece having a temperature in the range of 2200.degree. F.-2350.degree. F. than the same work piece which is heated only into the range of 1400.degree. F.-1800.degree. F. The increased force required to shape the work piece in warm forging as contrasted to conventional forging requires that the material of which the warm forging die is composed be very strong to resist the forging forces, very resistant to abrasion, and have high ductility and impact strength to resist the forging loads which are characteristically applied at a high strain rate with greater frequency than are encountered in conventional forging.